io_storage: use unsigned int for index
io_storage: use unsigned int for index

This change avoids warning about type conversion.

Signed-off-by: Yann Gautier <e753f6969c288905606e3c07464886d4f1fc1be0@st.com>
Signed-off-by: Etienne Carriere <f21bc5b8f3ea1962eb45af359f1abea6578cab94@st.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,lsigithub/arm-trusted-firmware_public